A new method was established to determine the orientation of individual grains on the specimen surface of commercial pure iron by analyzing the characteristic etch pits developed on its surface. This method was applied to determine the crystallographic configurations of crack nucleation sites and neighboring grains on the specimens tested under completely reversed plane bending fatigue stress. it was concluded that fatigue cracks nucleated along grain boundaries, and at each nucleation site, there were two primary slip systems intersecting approximately at right angle, in which {110} plane of one primary slip system was found to be parallel to the grain boundary and a slipping motion of another primary slip system was completely blocked by the former.
